Control circuit, resonance circuit, electronic device, control method, control program, and semiconductor element

ABSTRACT

A control circuit, resonant circuit, electronic device, control method, control program, and a semiconductor element, which enable a circuit to be measured and tuned within a short time even in consideration of a time constant when a control voltage is applied to a variable capacitance capacitor. A control circuit for a variable capacitance capacitor includes: a digital-analog converter that outputs a control voltage consisting of a variable DC voltage; the variable capacitance capacitor that has a capacitance varying with an application of the control voltage; a phase detector that acquires a characteristic of a circuit containing the variable capacitance capacitor; an analog-digital converter that subjects an analog signal from the phase detector to a digital conversion; a comparing section that compares a target value with a detected value; and a control section that sets the control voltage for the digital-analog converter on the basis of the comparison result.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control circuit for a variablecapacitance element whose capacitance varies with the application of aDC voltage, a resonant circuit including the control circuit, anelectronic device, a control method for the variable capacitanceelement, a control program, and a semiconductor element having thecontrol program. This application is based upon and claims the benefitof priority from the Japanese Patent Application No. 2013-146252 filedin Japan on Jul. 12, 2013, the entire contents of which are incorporatedherein by reference.

2. Description of Related Art

Noncontact communication technology utilizing electromagnetic inductionhas been increasingly applied to IC cards, including FeliCa™, Mifare™,and NFC (Near Field Communication). The application of this noncontactcommunication technology is now spreading even to noncontact charging(power feeding) techniques represented by a Qi format, for example.

When noncontact communication is conducted between a noncontact IC cardand a reader/writer as well as when noncontact charging in which arelatively large amount of electric power is fed or received isperformed, the signal transmission or the power feeding utilizeselectromagnetic coupling and a magnetic resonance phenomenon. In suchcoupling systems, the resonant frequency matching between the resonantcircuits on the transmitting and receiving sides is directly linked to adecrease in the number of transmission errors or an improvement in thetransmission efficiency. On the other hand, the resonant frequencies ofthe resonant circuits on the transmitting and receiving sides may varyand fluctuate due to various factors. The capacitances of capacitors andthe inductances of coils used in a resonant circuit have initialmanufacturing variations, and these variation ranges may be widenedbecause these capacitors and coils each have temperature characteristicsaccording to heat generated during an operation and a surroundingtemperature change. Furthermore, the resonant frequency may varydepending on a placement condition of a resonant circuit mounted in atransmitter or a receiver used. A relative positional relationshipbetween the resonant circuits on the transmitting and receiving sidesmay also influence the transmitting condition.

The resonant frequencies of constituent elements of a resonant circuitare individually tuned before shipping so that the characteristics,including temperature characteristics, of the constituent elements canfall within predetermined ranges. It is, however, extremely difficult todesign the circuit in consideration of varying resonant frequencies whenthe circuit is used. Even when certain levels of initial variations inelements are recognized, excessively reducing these initial variationscould not be preferred, because the excessive reduction in thevariations may lead to an increase in a material cost and complexity ofa manufacturing process.

For the purpose of correcting a variation in a resonant frequency of aresonant circuit and reducing an influence of the variation in theresonant frequency in use, techniques have been studied to automaticallytune the resonant frequency on each of the transmitting and receivingsides or automatically tune the resonant frequencies on both thetransmitting and receiving sides.

To give some examples, Patent Literature 1 describes a method in whichresonant capacitors are mounted in a resonant circuit and asemiconductor switch is used to tune the capacitances of the resonantcapacitors such that the resonant circuit has a desired resonantfrequency. Patent Literature 2 describes a method in which a variablecapacitance capacitor having a ferroelectric thin film is used as aconstituent resonant capacitor in a resonant circuit and the capacitanceof the resonant capacitor is controlled using an external DC bias suchthat the resonant circuit has a desired resonant frequency. Moreover,Patent Literature 2 describes a method in which the phase differencebetween input and output signals of a resonant coil is used to sense thetuned resonant frequency of the resonant circuit and the resonantfrequency can thereby be tuned easily without detecting the peak of ananalog signal.

Patent Literature 1: JP 2008-160312 A

Patent Literature 2: JP 2012-099968 A

BRIEF SUMMARY OF THE INVENTION

In the technique described in Patent Literature 1, the resonantfrequency needs to be tuned discretely. Therefore, it may be difficultto tune the resonant frequency to a desired one, for example whenconstituent elements of the resonant circuit have considerable initialvariations.

The technique described in Patent Literature 2 enables a resonantfrequency to be continuously tuned, advantageously obtaining a desiredresonant frequency easily. In the technique described in PatentLiterature 2, however, when the target of the resonant frequency issearched for, a control voltage is applied to the variable capacitancecapacitor while being elevated in a stepwise manner with the stepvoltage widths fixed. Therefore, as the number of steps for the stepvoltage width increases, the target can be acquired with higheraccuracy, but the tuning time is disadvantageously prolonged because thetuning time is proportional to the number of steps.

In addition to the sequential search method in which a control voltageis elevated in a stepwise manner with the step widths fixed, a dichotomymethod is known. In this dichotomy method, a target is narrowed down byfirst changing a control voltage in wide steps and then sequentiallyhalving the range of the control voltage in steps. Employing thedichotomy method could be expected to shorten the time for measuring andtuning the resonant frequency.

In addition to supplying transmission and reception AC signals, a DCbias needs to be applied to the variable capacitance capacitor in aresonant circuit whose resonant frequency is tunable, in order to varythe capacitance. To apply a DC voltage to the variable capacitancecapacitor separately from an AC signal, high-value bias resistors for ACblocking need to be placed at terminals of the variable capacitancecapacitor. However, if a DC control voltage is applied to the variablecapacitance capacitor in order to tune the resonant frequency, thecapacitance of the variable capacitance capacitor and the bias resistorsfor AC blocking may create a long time constant. In this case, it isnecessary to measure the resonant frequency after the lapse of a waittime that is sufficiently longer than the time constant. This is becausethe resonant frequency may be unable to be measured accurately beforethe control voltage applied to the variable capacitance capacitor isstabilized. Reserving a long wait time enables the resonant frequency tobe measured accurately but may have a problem in that a time forconverging to the target is prolonged in accordance with the number ofsteps.

An object of the present invention is to provide a control circuit, aresonant circuit, an electronic device, a control method, a controlprogram, and a semiconductor element, all of which enable a circuit tobe measured and tuned within a short period of time even inconsideration of a time constant when a control voltage is applied to avariable capacitance capacitor.

A control circuit, according to an embodiment of the present invention,for a variable capacitance element, which acts as means for addressingthe above problem, includes: a control voltage output section thatoutputs a control voltage generated from a variable DC voltage; avariable capacitance element that has a capacitance varying with anapplication of the control voltage; and a detection section thatacquires a characteristic of a circuit containing the variablecapacitance element. The detection section has a wait time between whenthe control voltage is applied to the variable capacitance element andwhen a measurement of the characteristic of the circuit is acquired. Thewait time is set to a plurality of values in accordance with the controlvoltage.

A resonant circuit according to an embodiment of the present inventionincludes a control circuit for a variable capacitance element and aresonant coil connected to the control circuit. The control circuitincludes: a control voltage output section that outputs a controlvoltage generated from a variable DC voltage; a variable capacitanceelement that has a capacitance that varies with an application of thecontrol voltage; and a detection section that acquires a characteristicof a circuit containing the variable capacitance element. The detectionsection has a wait time between when the control voltage is applied tothe variable capacitance element and when a measurement of thecharacteristic of the circuit is acquired. The wait time is set to aplurality of values in accordance with the control voltage.

An electronic device according to an embodiment of the present inventionincludes a control circuit for a variable capacitance element. Thecontrol circuit includes: a control voltage output section that outputsa control voltage generated from a variable DC voltage; a variablecapacitance element that has a capacitance varying with an applicationof the control voltage; and a detection section that acquires acharacteristic of a circuit containing the variable capacitance element.The detection section has a wait time between when the control voltageis applied to the variable capacitance element and when a measurement ofthe characteristic of the circuit is acquired. The wait time is set to aplurality of values in accordance with the control voltage.

A control method, according to an embodiment of the present invention,for a variable capacitance element includes: setting a capacitance of avariable capacitance element by applying a control voltage with acontrol voltage output section that outputs a variable DC voltage; andmeasuring a characteristic of a circuit containing the variablecapacitance element with a detection section that detects thecharacteristic of the circuit containing the variable capacitanceelement. The detection section has a wait time between when the controlvoltage is applied to the variable capacitance element and when ameasurement of the characteristic of the circuit is acquired. The waittime is set to a plurality of values in accordance with the controlvoltage.

According to an embodiment of the present invention, a control programfor a variable capacitance element is a control program that has stepsto be executable by a computer. The computer includes a storage sectionthat stores a program and a processing section that expands and executesthe stored program. The control program includes the steps of: setting acapacitance of a variable capacitance element by applying a controlvoltage with a control voltage output section that outputs a variable DCvoltage; and acquiring a characteristic of a circuit containing thevariable capacitance element with a detection section that detects thecharacteristic of the circuit containing the variable capacitanceelement. The detection section has a wait time between when the controlvoltage is applied to the variable capacitance element and when ameasurement of the characteristic of the circuit is acquired. The waittime is set to a plurality of values in accordance with the controlvoltage.

A semiconductor element according to an embodiment of the presentinvention includes a storage section that stores a control program for avariable capacitance element. The semiconductor element further includesa processing section that executes the control program stored in thestorage section.

With the present invention, a wait time between when a control voltageis applied to a variable capacitance element and when a measurement of acharacteristic of a circuit is acquired is set to a plurality of valuesin accordance with the control voltage. Thus, the present inventionenables a measurement of the characteristic of the circuit to beacquired within a short wait time.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary control circuit for a variablecapacitance capacitor according to an embodiment employing the presentinvention.

FIG. 2A is a circuit diagram that is an exemplary circuit configurationof the variable capacitance capacitor.

FIG. 2B is a circuit diagram of the variable capacitance capacitor,including a signal source connected to the terminals.

FIG. 3 is a graph that indicates the comparison between times formeasuring a target value with a dichotomy method and a sequential searchmethod.

FIG. 4A is a graph that indicates how many steps are required to searchfor a target value with the sequential search method.

FIG. 4B is a graph that indicates how many steps are required to searchfor a target value with the dichotomy method.

FIG. 5 is a graph that displays a waveform of measurements of a currentflowing through the variable capacitance capacitor in the circuit ofFIG. 2B, and is a graph that charts a measurement of a time period overwhich the current converges into a constant value when step voltagewidths of a control voltage applied to the DC input terminals of thevariable capacitance capacitor are changed.

FIG. 6 is a graph that displays a waveform of measurements of a currentflowing through the variable capacitance capacitor in the circuit ofFIG. 2B with a time constant set to twice of the time constant in FIG.5, and is a graph that charts a measurement of a time period over whichthe current converges into a constant value when the step voltage widthsof the control voltage applied to the DC input terminals of the variablecapacitance capacitor are changed.

FIG. 7A is a graph that indicates the comparison between a measurementtime for searching for a target value with an improved dichotomy methodand a measurement time for searching for the same target value with thesequential search method.

FIG. 7B is a graph that indicates the comparison between measurementtimes for searching for the same target value with the improveddichotomy method of FIG. 7A (dichotomy method 1) and a further improveddichotomy method (dichotomy method 2).

FIG. 8 is a graph indicating that the control voltage has different timeconstants at its rising and falling portions.

FIG. 9 is an exemplary flowchart for performing an operation of thecontrol circuit and is a flowchart for performing initial setting.

FIG. 10 is an exemplary flowchart for performing the operation of thecontrol circuit and is a flowchart in which the control circuitinitialized in accordance with the flowchart of FIG. 9 searches for atarget value with the improved dichotomy method.

FIGS. 11A to 11C are each a graph that indicates the comparison betweenmeasurement times for reaching a target value with an improved dichotomymethod and a normal dichotomy method. In FIG. 11A, the improveddichotomy method (dichotomy method 1) is compared with the normaldichotomy method; in the dichotomy method 1, the maximum step voltagewidth is set to ½ of the maximum of the control voltage, and a wait timefor other step voltage widths is uniformly set. In FIG. 11B, the waittime is set in proportion to step voltage widths as an improveddichotomy method 2. In FIG. 11C, the wait time is set in proportion tostep voltage widths as an improved dichotomy method 3, and the wait timefor the falling portion of the control voltage is set to be shorter thanthe wait time for the rising portion of the control voltage.

FIG. 12 is a block diagram of an exemplary noncontact communicationsystem.

FIG. 13 is a block diagram of an exemplary noncontact charging system.

DETAILED DESCRIPTION OF THE INVENTION

Some embodiments of the present invention will be described below indetail with reference to the accompanying drawings. Needless to say, thepresent invention is not limited to embodiments that will be describedbelow, and these embodiments can be modified in various ways withoutdeparting from the spirit of the present invention.

The description will be given in the following order.

1. Exemplary Configuration of Control Circuit that Controls ResonantFrequency

2. Principle of Operation of Control Circuit 3. Operation Sequence ofControl Circuit 4. Exemplary Operation of Control Circuit 5. ExemplaryConfiguration of Electronic Device

1. Exemplary Configuration of Control Circuit that Controls ResonantFrequency

As illustrated in FIG. 1, a control circuit (referred to below simply asa control circuit) 1 according to an embodiment employing the presentinvention controls a resonant frequency. The control circuit 1 includesa resonating section having a variable capacitance capacitor (VC1) 2that forms a resonant capacitor, a resonant coil (L1) 4 connected inparallel to the variable capacitance capacitor (VC1) 2, and a seriesresonant capacitor (C1) 3 connected in series to the resonant coil (L1)4. In addition, the control circuit 1 includes: a digital-analogconverter (referred to below simply as a DAC) 11 that applies a controlvoltage (Vcont) 19 to the variable capacitance capacitor (VC1) 2; aphase detector 12 that receives input and output signals for theresonating section and detects the phase difference therebetween; ananalog-digital converter (referred to below simply as an A/DC) 13 thatconverts an output signal (Phase_det) 17 forming a phase differencesignal detected by the phase detector 12 into a digital signal; and acomparing section 14 that compares a digital value of the output signal(Phase_det) 17 with a preset target value (Target) 18.

The control circuit 1 further includes a control section 10 that directsthe DAC 11 to set a next control voltage 19 to be applied to thevariable capacitance capacitor (VC1) 2 via the DAC 11, on the basis ofthe comparison result from the comparing section 14. When the DAC 11sets a voltage value of the control voltage 19, the control section 10directs the DAC 11 to output the control voltage 19 after the elapse ofa preset wait time. The control section 10 preferably has a processingsection 10 a formed of an APU, and the processing section 10 apreferably reads and executes individual steps in a program stored in amemory 10 b. The processing section 10 a and the memory 10 b may beimplemented using an integrated semiconductor device. Alternatively, theprocessing section 10 a may be implemented using an MPU or amicrocontroller, and the memory 10 b may be implemented using a ROM, aRAM, a magnetic memory, or a combination thereof.

Before an input signal enters a resonating section of the controlcircuit 1, the input signal preferably passes through a filter circuitformed of a coil (Lf) 5 and a capacitor (Cf) 6 so that only apredetermined frequency component passes therethrough.

When an AC signal having a frequency f0 enters the control circuit 1 viaa signal input terminal (TX) 8, the AC signal enters the resonatingsection constituted by the series resonant capacitor (C1) 3, thevariable capacitance capacitor (VC1) 2, and the resonant coil (L1) 4.Then, the control voltage (Vcont) 19 output from the DAC 11 is adjustedsuch that a resonant frequency f1 of the resonating section matches thefrequency f0 of the input signal. The frequency difference between thefrequencies f0 and f1 is measured with the phase detector 12. Signals toenter the phase detector 12 are an input signal (REF) 15 to enter theresonating section and an output signal (MONITOR) 16 formed of a currentsignal that has flown through the resonating section. The output signal(MONITOR) 16 is converted into a voltage signal by a current sensingresistor (R6) 7 connected to the resonating section, and this voltagesignal is detected. The phase detector 12 outputs the output signal(Phase_det) 17 in accordance with the phase difference between the inputsignal (REF) 15 and the output signal (MONITOR) 16. Then, the controlsection 10 adjusts the control voltage (Vcont) 19 to be output from theDAC 11 until the output signal (Phase_det) 17 and the target value(Target) 18 are compared and match each other or a compared valuebecomes equal to or less than the resolution of the A/DC 13.

FIGS. 2A and 2B illustrate exemplary configurations of the variablecapacitance capacitor (VC1) 2. As illustrated in FIG. 2A, the variablecapacitance capacitor (VC1) 2 has four variable capacitance elements (C2to C5) 22 to 25 connected in series between two AC input terminals (AC1and AC2) 20 a and 20 b. The number of variable capacitance elementsconnected in series should be selected appropriately in accordance witha withstand voltage required for a circuit, and may be more than four orless than four. Alternatively, a single variable capacitance element maybe used. The variable capacitance elements (C2 to C5) 22 to 25 typicallyhave the same capacitance; however, it is obvious that there is nolimitation on the capacitances of the variable capacitance elements (C2to C5) 22 to 25.

The variable capacitance capacitor (VC1) 2 may be fabricated bylaminating a plurality of layers, each of which includes a ferroelectricthin film made of barium titanate, for example, and metal electrodesformed on the ferroelectric thin film with vapor deposition.

The capacitance of the variable capacitance capacitor (VC1) 2 can varywith the application of a DC bias to the variable capacitance elements(C2 to C5) 22 to 25. The DC bias voltage is applied between DC inputterminals (DC1 and DC2) 21 a and 21 b. In this case, the same DC biasvoltage cannot be applied directly between the end electrodes of each ofthe variable capacitance elements (C2 to C5) 22 to 25, because theelectrodes may be shorted. Therefore, the DC bias voltage is appliedbetween the end electrodes of each variable capacitance element viaimpedance elements; each impedance element has an impedance that issufficiently larger than the impedance of each variable capacitanceelement which corresponds to the frequency of the AC signal applied tothe variable capacitance elements (C2 to C5) 22 to 25. Morespecifically, the DC input terminals (DC1 and DC2) 21 a and 21 b areconnected to the end electrodes of variable capacitance element (C2) 22via high-value resistors (R2 and R1) 27 and 26, respectively. Likewise,the DC input terminals (DC1 and DC2) 21 a and 21 b are connected to theend electrodes of the variable capacitance element (C3) 23 via thehigh-value resistors (R2) 27 and a high-value resistors (R3) 28,respectively. The DC input terminals (DC1 and DC2) 21 a and 21 b areconnected to the end electrodes of the variable capacitance element (C4)24 via a high-value resistor (R4) 29 and the high-value resistor (R3)28, respectively. The DC input terminals (DC1 and DC2) 21 a and 21 b areconnected to the end electrodes of the variable capacitance element (C5)25 via the high-value resistor (R4) 29 and a high-value resistor (R5)30. With this connection, the same level of DC bias voltages are appliedto the end electrodes of all the variable capacitance elements (C2 toC5) 22 to 25.

As illustrated in FIG. 2B, the AC input terminals (AC1 and AC2) 20 a and20 b of the variable capacitance capacitor (VC1) 2 are connected to theAC signal source (VAC) 31. To measure the current flowing through thevariable capacitance capacitor (VC1) 2, the current sensing resistor(R6) 7 may be connected between the AC input terminal (AC2) 20 b and theAC signal source (VAC) 31. The output impedance of the AC signal source(VAC) 31 corresponds to a resistor (R8) 32. The DC input terminals (DC1and DC2) 21 a and 21 b of the variable capacitance capacitor (VC1) 2 areconnected to a control voltage source (Vcont) 33 that outputs a variableDC bias voltage. In FIG. 2B, a positive voltage potential is applied tothe DC input terminal (DC1) 21 a, whereas a negative voltage potentialis applied to the DC input terminal (DC2) 21 b. However, it is obviousthat the connection may be reversed because the variable capacitancecapacitor has no polarity, and thus the connection can be set inaccordance with an application circuit.

An exemplary configuration of a control circuit to be mounted in anoncontact communication device, for example, has been described; thecontrol circuit controls the resonant frequency of a resonant circuithaving a variable capacitance capacitor as a resonant capacitor.However, it is obvious that this configuration is also applicable toother control circuits that control the capacitance of the variablecapacitance capacitor with a control voltage via an AC blocking resistorand tunes characteristics of a circuit including the variablecapacitance capacitor. When the control circuit is used as that foranother control circuit, a characteristic value to be detected may be aresonant frequency, a circuit impedance, or some other circuitcharacteristic. A configuration of a detection circuit can be selectedand configured depending on a circuit characteristic to be detected.

2. Principle of Operation of Control Circuit (1) Principle of Operationof Dichotomy Method

FIG. 3 conceptually illustrates the difference between the measuringtimes when the target value of a control voltage is searched for withthe dichotomy method and the sequential search method. FIG. 3illustrates an exemplary case where the target value is in the range of2.5 V to 3 V. When the sequential search method represented by a brokenline is used, a time period corresponding to 15 time units (relativetime units) is required. In contrast, when the dichotomy method is used,a time period corresponding to 5 time units is required. Thus, thedichotomy method makes it possible to search for a target value within ⅓of the measuring time for the sequential search method.

When the target value is in the vicinity of 2.7 V as illustrated in FIG.3, the target value is searched for in the following manner with thedichotomy method.

It is determined that the target value is higher or lower than 1.5 Vthat is ½ of the maximum voltage, or 3 V. If the target value is higher,the target value is considered to fall within the range of 1.5 V to 3 V,and the determination range shifts to the next one. The nextdetermination range is the range of 1.5 V to 2.25 V (=1.5 V+(3 V−1.5V)/2), and it is determined whether the target value is higher or lowerthan 2.25 V. If it has been determined that the target value is higherthan 2.25 V, the target value falls within the range of 2.25 V to 2.625V (=2.25 V+(3 V−2.25 V)/2), and it is determined whether the targetvalue is higher or lower than 2.625 V. If the target value has beenhigher than 2.625 V, the next determination range is the range of 2.625V to 2.8125 V (=2.625 V+(3 V−2.625 V)/2), and the target value isconcluded to fall within the range of 2.8125 V to 3 V.

In the above way, the search range of the control voltage is halved insteps so that the range in which the target value is present isnarrowed. Consequently, it is possible to reach the target value througha small number of steps and within a short time period.

FIGS. 4A and 4B conceptually illustrate how to reach a target value(Vtarget) on an actually measured characteristic curve (e.g., the phasedetection voltage (Phase_det) associated with the control voltage(Vcont) in the control circuit of FIG. 1), together with a differencebetween the sequential search method and the dichotomy method.

In the sequential search method, as illustrated in FIG. 4A, the phasedetection voltage, which is a target to be measured, is measured in stepvoltage widths (e.g., in steps of an LSB of the DAC) obtained by equallydividing the maximum value of the control voltage, or 3 V. In FIG. 4A,the step increment is repeated nine times and then the target valueVtarget is reached.

In the dichotomy method, as illustrated in FIG. 4B, the measurementstarts at 1.5 V, which is ½ of the maximum value of the control voltage,or 3 V, and the target value Vtarget is reached through four steps.

(2) Problem with Dichotomy Method

As illustrated in FIGS. 1 and 2, when tuning the capacitance of thevariable capacitance capacitor (VC1) 2, the control section 10 appliesthe control voltage (Vcont) 19 to the electrodes of the variablecapacitance elements (C2 to C5) 22 to 25 via the high-value resistors(R1 to R5) 26 to 30. In this case, when the control voltage (Vcont) 19is applied between the DC input terminals (DC1 and DC2) 21 a and 21 b ina stepwise manner, a considerable time period is required for thecontrol voltage (Vcont) 19 to reach a desired constant voltage. This isbecause the change in the control voltage (Vcont) 19 depends on a timeconstant determined by the product of the capacitances of variablecapacitance elements (C2 to C5) 22 to 25 and the resistances of theresistors for AC blocking (R1 to R5) 26 to 30. Assuming that thevariable capacitance elements (C2 to C5) 22 to 25 have the samecapacitance (C2 to C5=C) and the resistor for AC blocking (R1 to R5) 26to 30 have the same resistance (R1 to R5=R), the time constant τ isequal to 2CR because the resistor for AC blocking is connected acrosseach variable capacitance element.

In general, when a variable capacitance capacitor having a ferroelectricthin film is used in a resonant circuit in a noncontact communicationsystem or a transmitting/receiving antenna of a noncontact chargingsystem, the resonant circuit includes a plurality of variablecapacitance elements connected in series in order to sufficientlyincrease an in-use withstand voltage. If the resistors for AC blocking(R1 to R5) 26 to 30 each have a low resistance, an AC signal that hasentered the AC input terminals (AC1 and AC2) 20 a and 20 b may flow outto the DC input terminals (DC1 and DC2) 21 a and 21 b. Furthermore, theAC signal may flow while bypassing the resistors for AC blocking,thereby increasing a loss. The increase in the loss may result in thelowering of the Q (quality factor) of the resonant circuit.

For the above reason, the resistance of each of the resistors for ACblocking (R1 to R5) 26 to 30 is set to a large value. As a result, r=2CRincreases. The increase in τ may result in an extension of a measuringtime, even when the target value is searched for with the dichotomymethod.

(3) Improvement in Dichotomy Method

To address the above problem, the inventor of the present inventionfound a predetermined relationship between a time constant determined bythe capacitance of a variable capacitance capacitor and the resistanceof a resistor for AC blocking and a step voltage width to be applied.From the above relationship, then, the inventor derived improveddichotomy methods that enable short-time measurement.

FIG. 5 illustrates waveforms of voltages across the current sensingresistor (R6) 7 (i.e., the waveforms of currents flowing through thevariable capacitance capacitor (VC1) 2) when the control voltage (Vcont)in the circuit of FIG. 2B is changed in a stepwise manner. The waveformsof FIG. 5 are obtained when the step voltage width of the controlvoltage (Vcont) is changed to 0.375 V, 0.75 V, 1.5 V, and 3.0 V. As thestep voltage width increases, a time for converging to a constantvoltage is prolonged. Assuming that a range in which the control voltage(Vcont) 33 can be recognized as a constant voltage (initial current) isconsidered to be a voltage value considering a detection accuracy Vtolof a measuring system and a time period between when the application ofa step voltage starts and when the control voltage (Vcont) 33 becomes aconstant voltage is defined as a stabilization time, a stabilizationtime difference Δt is present between the stabilization times when thestep voltage width is 0.375 V and 3.0 V.

FIG. 6 illustrates voltage waveforms across the current sensing resistor(R6) 7 (i.e., current waveforms flowing through the variable capacitancecapacitor (VC1) 2) when the control voltage (Vcont) in the circuit ofFIG. 2B is changed in a stepwise manner with a time constant that istwice as long as the time constant of FIG. 5. A stabilization timedifference Δt′ in step voltage width is remarkably greater than thestabilization time difference Δt in the case of FIG. 5.

In the cases of FIGS. 5 and 6, the stabilization time associated with astep voltage width is proportional to the step voltage width. When thestep voltage width decreases, the stabilization time can be shortened.Therefore, the control circuit of the present invention sets astabilization time in accordance with a time constant; the time constantis calculated from the capacitance of a variable capacitance elementused in a circuit and the resistance of an AC blocking resistor viawhich a DC bias voltage is applied to the variable capacitance element.After the elapse of the stabilization time, the control circuit measurescircuit characteristics, such as a resonant current, and then sets thestabilization time in accordance with the step voltage width of thecontrol voltage. When the step voltage width is narrow, the controlcircuit attempts to decrease a time for reaching a target value byshortening the stabilization time. In this case, the stabilization timeis preferably set to substantially 5 times the time constant (≈5τ=10CR).

FIG. 7A indicates the difference in time for reaching a target valuebetween an improved dichotomy method (improved dichotomy method 1) andthe sequential search method; the improved dichotomy method 1 considersthe time constant determined by the capacitance of the variablecapacitance element and the resistance of the AC blocking resistor. Theimproved dichotomy method 1 represented by the solid line makes itpossible to shorten a time for reaching the target value by an amountproportional to a decreased number of steps, in comparison with thesequential search method represented by the broken line.

When a normal dichotomy method is initiated at 0 V, the initial stepwidth is set to the maximum, or 3 V. When the improved dichotomy methodis initiated, the initial step width is set to ½ of the maximum voltage.By setting the step voltage width to ½ of the maximum, the stabilizationtime can be shortened, because the stabilization time is proportional tothe step voltage width.

In the improved dichotomy method 1, when the time constant, which isdetermined by the capacitance of the variable capacitance element andthe resistance of the AC blocking resistor, is long, it may be difficultto sufficiently shorten a measuring time. However, by changing thestabilization time in accordance with each step voltage width of thecontrol voltage, the measuring time can be shortened. As indicated inFIG. 7B, an improved dichotomy method 2 (indicated by a solid line) inwhich the stabilization time is set in proportion to a step voltagewidth of a control voltage can shorten a time for reaching a targetvalue, in comparison with the improved dichotomy method 1 indicated bythe broken line.

When a step-like control voltage is applied to DC input terminals (DC1and DC2) 21 a and 21 b of the variable capacitance capacitor 2 asillustrated in FIG. 8, this control voltage exhibits different timeconstants at a (rising) portion to which the increasing step voltage isapplied and at a (falling) portion to which the decreasing step voltageis applied. More specifically, the time constant at the rising portionof the control voltage is longer than that at the falling portionthereof. Therefore, the stabilization time is changed depending on thedirection in which the control voltage changes in a stepwise manner withthe above improved dichotomy methods, whereby the measuring time can befurther shortened.

3. Operation Sequence of Control Circuit

FIGS. 9 and 10 are exemplary flowcharts of a sequence for achieving theabove operation of the control circuit. In the flowchart illustrated inFIG. 9, an operation sequence of the control circuit is initially set,and a tuning mode in which the resonant frequency is tuned is set. Theflowchart of FIG. 10 displays an operation sequence in the tuning mode,more specifically an operation sequence of an improved dichotomy method.

As illustrated in FIG. 9, at Step S1, the control section 10 startsinitial setting in the tuning mode. The following steps will beperformed by the operation of the control section 10. It is preferablethat programs containing the respective steps be stored in the memory 10b, and read and executed in order by the processing section 10 a.

At Step S2, the control section 10 sets the initial number n of stepsfor the control voltage (Vcont) 19 to 1. At Step S3, the control section10 sets the output of the DAC 11 that outputs the control voltage(Vcont) 19 to 0 V.

At the next step, the DAC 11 sets the maximum of a wait time betweenwhen the control voltage (Vcont) 19 is applied and when the resonantfrequency is measured. More preferably, at Step S4, the control section10 sets the maximum (tmax (P)) of a wait time when the control voltage(Vcont) 19 is changed in the increasing direction. At Step S5, thecontrol section 10 sets the maximum (tmax (N)) of a wait time when thecontrol voltage (Vcont) 19 is changed in the decreasing direction. Inthis case, the stabilization time (≈2τ=10CR) or a value obtained bymultiplying the stabilization time by a preset margin may be used as thewait time.

At Step S6, the control section 10 sets the target value (Target) 18 tobe searched for. At Step S7, the control section 10 sets an OK range as±allowable range voltage for the target value (Target) 18.

As illustrated in FIG. 10, the control circuit 1 that has been subjectedto the initial setting of FIG. 9 and entered the tuning mode willoperate in the following sequence. The control circuit 1 operates byvirtue of the effect of the control section 10, similar to the initialsetting.

This embodiment employs the configuration in which when the controlvoltage (Vcont) 19 increases, the output voltage (Phase_det) 17 of thephase detector 12 decreases. Thus, at Step S10, the control section 10determines whether the output voltage (Phase_det) 17 of the phasedetector 12 that has received the input signal (REF) 15 to enter theresonating section and the output signal (MONITOR) 16 output from theresonating section is lower than the target value (Target) 18. If theoutput voltage (Phase_det) 17 is higher than the target value (Target)18 that has been set upon the initial setting, the control section 10determines that the tuning is impossible and performs an error processat Step S11. If the output voltage (Phase_det) 17 is lower than thetarget value (Target) 18, the operation proceeds to Step S12 andsubsequent steps and starts the tuning operation.

At Step S13, the control section 10 sets the number n of steps for thecontrol voltage to 2n.

At Step S14, the control section 10 sets the step voltage in theincreasing direction to VCC/n. In this case, VCC denotes the maximum ofthe output voltage (control voltage Vcont) of the DAC 11.

At Step S15, the control section 10 sets the wait time that starts whenthe DAC 11 outputs the control voltage (Vcont) 19 to 2×tmax (P)/n.

At Step S16, if the output voltage (Phase_det) 17 of the phase detector12 falls within the target value (Target) 18 ±OK range, the controlsection 10 considers that the output voltage (Phase_det) 17 has reachedthe target value (Target) 18. So, at Step S17, the control section 10holds the current output voltage of the DAC 11 and terminates the tuningmode. If the output voltage (Phase_det) 17 of the phase detector 12falls outside the target value (Target) 18 ±OK range, the operationproceeds to the next step, or Step S18.

At Step S18, the control section 10 checks the magnitude relationshipbetween the output voltage (Phase_det) 17 of the phase detector 12 andthe target value (Target) 18. If the output voltage (Phase_det) 17 islower than the target value (Target) 18, the operation returns to StepS13. Then, the control section 10 newly sets the number of steps andhalves the search range, and repeats the operations of Steps S13 to S16. If the output voltage (Phase_det) 17 is higher than the target value(Target) 18, the operation proceeds to the next step, or Step S19. Sincethe output voltage (Phase_det) 17 of the phase detector 12 is invertedat Step S16, the control section 10 searches for the target value(Target) 18 in the deceasing direction of the step voltage at the nextstep, or Step S19.

At Step S19, the control section 10 sets the number n of steps to 2n. AtStep S20, the control section 10 sets the step voltage width in thedecreasing direction to −VCC/n. At Step S21, the control section 10 setsthe wait time to 2×tmax (N)/n.

After that, operation returns to Step S 16. Then, the control section 10repeats the above operation until the output voltage (Phase_det) 17 ofthe phase detector 12 falls within the target value (Target) 18 ±OKrange.

4. Exemplary Operation of Control Circuit

The measuring time was obtained when the control circuit 1 illustratedin FIG. 1 operated in accordance with the above operation flow. FIGS.11A to 11C each indicate the comparison between the measuring times whenthe target value was measured with a dichotomy method typically used andwhen the target value was measured with the above improved dichotomymethod.

FIG. 11A indicates the measuring time for the improved dichotomymethod 1. In this method, the wait time for the control voltage (Vcont)19 increases in proportion to the step voltage width of the controlvoltage (Vcont) 19, and by utilizing this property, the initial voltageis set to ½ of the maximum of the control voltage (Vcont). The waittimes for respective steps are the same as one another. In the flowchartof FIG. 10, both tmax (P) and tmax (N) are set to tmax at Steps S4 andS5, and n for the wait time is fixed to 1 at Steps S15 and S21. Thisinitial setting is the simplest and can decrease the number of steps inthe program. The improved dichotomy method 1 can halve the measuringtime in comparison with the normal dichotomy method.

FIG. 11B indicates an exemplary measurement with the improved dichotomymethod 2. The wait time is changed in steps for the control voltage(Vcont) 19. In the improved dichotomy method 2, both tmax (P) and tmax(N) are set to tmax at Steps S4 and S5 in the flowchart of FIG. 10, andthe wait times for Steps S15 and S21 are changed in accordance with thestep voltage width. Since the step voltage width of the control voltage(Vcont) 19 decreases from step to step in the dichotomy method, and inproportion to this, the wait time is also set to be shorter (Steps S13to S15 and Steps S19 to S21 in FIG. 10). The measuring time is decreasedto approximately ⅙ of the measuring time for the normal dichotomymethod.

FIG. 11C indicates the improved dichotomy method 3. The stabilizationtime is changed in steps for the control voltage (Vcont) 19, and thewait time for the falling portion of the control voltage (Vcont) 19 isset to be shorter than the rising portion thereof. The improveddichotomy method 3 can decrease the measuring time more than ⅙ of themeasuring time for the normal dichotomy method.

5. Exemplary Configuration of Electronic Device

The control circuit of the present invention is used for a resonantcircuit used in noncontact communicating devices, antenna circuits innoncontact charging devices and any other devices and control circuitsthat control a resonant frequency of such devices and circuits. Thecontrol circuit is used to tune a resonant frequency depending on ausage condition.

(1) Exemplary Configuration of Noncontact Communicating Device

A resonant circuit that includes resonant capacitors and resonant coilsis mounted in a noncontact communicating device, and is used tocommunicate with another noncontact communicating device in a noncontactmanner. The noncontact communicating device is a noncontactcommunication module 150 conforming to the NFC (Near FieldCommunication) or the like which is mounted, for example in a portablephone. The other noncontact communicating device is a reader/writer 140,for example in a noncontact communication system.

As illustrated in FIG. 12, the noncontact communication module 150includes a secondary antenna section 160 that has a resonant circuitformed of: a resonant capacitor including variable capacitancecapacitors; and a resonant coil. The noncontact communication module 150includes: a rectification section 166 that converts an AC signaltransmitted from the reader/writer 140 into DC power by rectifying theAC signal in order to use the AC signal as a power source for individualblocks; and a voltage regulator section 167 that generates voltagescorresponding to the blocks. The noncontact communication module 150includes: a demodulation section 164; a modulation section 163; and areception control section 165 that operate with the DC power suppliedfrom the voltage regulator section 167. In addition, the noncontactcommunication module 150 includes a system control section 161 thatcontrols operations of the entire module. When the secondary antennasection 160 receives a signal, the rectification section 166 convertsthe signal into DC power, and a demodulation section 164 demodulates thesignal. Then, the system control section 161 analyzes data transmittedfrom the reader/writer 140. The system control section 161 generatesdata to be transmitted from the noncontact communication module 150. Themodulation section 163 modulates the transmission data to generate asignal to be transmitted to the reader/writer 140. The secondary antennasection 160 transmits the signal. The reception control section 165tunes the resonant frequency of the secondary antenna section 160 inaccordance with the control sequence of the system control section 161.The reception control section 165 receives an input signal (REF) 115 forthe secondary antenna section 160 and an output signal (MONITOR) 116 forthe secondary antenna section 160, and compares the phases of thesesignals. As a result of comparing the input and output signals, thereception control section 165 controls a control voltage (Vcont) 119such that the resonant frequency of the secondary antenna section 160matches a target value, or the resonant frequency transmitted from theprimary antenna section 120.

The reader/writer 140 in the noncontact communication system includes aprimary antenna section 120 that includes a resonant circuit having avariable capacitance circuit made of a resonant capacitor and a resonantcoil. The reader/writer 140 includes: a system control section 121 thatcontrols operations of the reader/writer 140; a modulation section 124that modulates a transmission signal on the basis of an instruction fromthe system control section 121; and a transmission signal section 125that transmits, to the primary antenna section 120, a carrier signalmodulated by the transmission signal from the modulation section 124.Furthermore, the reader/writer 140 includes a demodulation section 123that demodulates the modulated carrier signal transmitted from thetransmission signal section 125.

Obviously, the reader/writer 140 may also be equipped with a function ofautomatically tuning a resonant frequency, which is similar to that ofthe noncontact communication module 150.

(2) Operation of Noncontact Communicating Device

The reader/writer 140 tunes the impedance matching with the primaryantenna section 120 on the basis of the carrier signal transmitted fromthe transmission signal section 125. The modulation section 124 employsa modulation scheme and an encoding scheme that may be used by generalreader/writers, examples of which are a Manchester encoding scheme andan ASK (Amplitude Shift Keying) modulation scheme. The carrier frequencyis typically 13.56 MHz.

A transmission/reception control section 122 monitors a transmissionvoltage and transmission current of the transmitted carrier signal, andtunes the impedance so as to ensure the impedance matching bycontrolling a variable voltage Vc of the primary antenna section 120.

When the reader/writer 140 transmits a signal, the secondary antennasection 160 in the noncontact communication module 150 receives thesignal, and then the demodulation section 164 demodulates the signal.The system control section 161 determines the content of the demodulatedsignal, and the system control section 161 generates a response signalon the basis of this result. The reception control section 165 tunes theresonant parameter and any other parameters of the secondary antennasection 160 on the basis of the voltage phase and current phase of thereception signal, thereby tuning the resonant frequency such that thereception condition becomes optimum. As described above, the receptioncontrol section 165 compares the phase of the input signal (REF) 115 ofthe reception signal with the phase of the output signal (MONITOR) 116,and tunes the resonant frequency by adjusting the control voltage(Vcont) 119 under the control of the system control section 161.

In the noncontact communication module 150, the modulation section 163modulates the response signal, and the secondary antenna section 160transmits the modulated response signal to the reader/writer 140. In thereader/writer 140, the demodulation section 123 demodulates the responsesignal received by the primary antenna section 120, and the systemcontrol section 121 performs a necessary process on the basis of thedemodulated content.

(3) Exemplary Configurations of Noncontact Charging Device and PowerReceiving Device

A control circuit and a resonant circuit that employ the presentinvention can be implemented using a power receiving device 190: thepower receiving device 190 is contained in a portable terminal such as aportable phone and has a secondary battery that is chargeable in anoncontact manner with a noncontact charging device 180. There is nospecific limitation on this noncontact charging system; however, anelectromagnetic induction system or a magnetic resonance system, forexample, is applicable.

FIG. 13 illustrates an exemplary configuration of a noncontact chargingsystem; the noncontact charging system includes: the power receivingdevice 190 for a portable terminal, for example, which employs thepresent invention; and the noncontact charging device 180 that chargesthe power receiving device 190 in a noncontact manner.

The power receiving device 190 has substantially the same configurationas the above noncontact communication module 150. The configuration ofthe noncontact charging device 180 is substantially that same as theconfiguration of the above reader/writer 140. Therefore, blocks thathave the same functions as corresponding blocks in the reader/writer 140or the noncontact communication module 150 illustrated in FIG. 12 aredenoted by the same reference marks. In many cases, the reader/writer140 supports a transmission or reception carrier frequency of 13.56 MHz,whereas in some cases, the noncontact charging device 180 supports atransmission or reception carrier frequency of one hundred kHz toseveral hundred kHz.

The noncontact charging device 180 tunes the impedance matching with aprimary antenna section 120, on the basis of a carrier signaltransmitted from a transmission signal section 125.

A transmission/reception control section 122 monitors a transmissionvoltage and transmission current of the transmitted carrier signal, andtunes the impedance so as to ensure the impedance matching bycontrolling a variable voltage Vc of the primary antenna section 120.

In the power receiving device 190, a secondary antenna section 160receives a signal, and a rectification section 166 rectifies the signal.A battery 169 is charged with the rectified DC voltage under the controlof the charge control section 170. Even if the secondary antenna section160 receives no signals, the charge control section 170 can be driven byan external power source 168, such as an AC adaptor, so that the battery169 is charged.

When the noncontact charging device 180 transmits a signal, thesecondary antenna section 160 receives this signal, and then thedemodulation section 164 demodulates the signal. A system controlsection 161 determines the content of the demodulated signal, and thesystem control section 161 generates a response signal on the basis ofthis result. A reception control section 165 adjusts a control voltage(Vcont) 119 on the basis of a voltage phase (input signal (REF) 115) anda current phase (output signal (MONITOR) 116) of the reception signal,thereby tuning the capacitance of the variable capacitance capacitor inthe secondary antenna section 160. In this way, the resonant frequencyis tuned such that the reception condition becomes optimum.

GLOSSARY OF DRAWING REFERENCES

1 . . . Control Circuit, 2 . . . variable capacitance capacitor, 3 . . .series resonant capacitor, 4 . . . resonant coil, 5 . . . filter coil, 6. . . filter capacitor, 7 . . . current sensing resistor, 8 . . . signalinput terminal, 10 . . . control section, 11 . . . digital-analogconverter, 12 . . . phase detector, 13 . . . digital-analog converter,14 . . . comparing section, 15 . . . input signal REF), 16 . . . outputsignal (MONITOR), 17 . . . output signal (Phase_det), 18 . . . targetvalue (Target), 19 . . . control voltage (Vcont), 20 a, 20 b . . . ACinput terminals, 21 a, 21 b . . . DC input terminals, 22-25 . . .variable capacitance elements, 26-30 . . . high-value resistors, 31 . .. AC signal source, 32 . . . output impedance, 33 . . . control voltagesource

1. A control circuit for a variable capacitance element, comprising: acontrol voltage output section outputting a control voltage consistingof a variable DC voltage; a variable capacitance element having acapacitance varying with an application of the control voltage; and adetection section acquiring a characteristic of a circuit containing thevariable capacitance element, wherein the detection section having await time between when the control voltage is applied to the variablecapacitance element and when a measurement of the characteristic of thecircuit is acquired, and the wait time being set to a plurality ofvalues in accordance with the control voltage.
 2. The control circuitaccording to claim 1, wherein the wait time is changed depending on adisplacement of the control voltage.
 3. The control circuit according toclaim 1, wherein the wait time is changed depending on the direction inwhich the control voltage is displaced.
 4. The control circuit accordingto claim 1, wherein the wait time is changed depending on a displacementof the control voltage and the direction in which the control voltage isdisplaced.
 5. The control circuit according to claim 2, wherein the waittime is made proportional to the displacement of the control voltage. 6.The control circuit according to claim 3, wherein the wait time when thecontrol voltage is deceased is set to be shorter than that when thecontrol voltage is increased.
 7. The control circuit according to claim1, wherein the control voltage output section is connected to thevariable capacitance element through an AC-signal-blocking impedanceelement.
 8. The control circuit according to claim 1, wherein thecharacteristic of the circuit is a resonant frequency or an impedance ofthe circuit.
 9. A resonant circuit comprising: the control circuitaccording to claim 1; and a resonant coil connected to the controlcircuit.
 10. An electronic device comprising the control circuitaccording to claim
 1. 11. A control method for a variable capacitanceelement, comprising: setting a capacitance of a variable capacitanceelement by applying a control voltage with a control voltage outputsection that outputs a variable DC voltage; and measuring acharacteristic of a circuit containing the variable capacitance elementwith a detection section that detects the characteristic of the circuitcontaining the variable capacitance element, wherein the detectionsection having a wait time between when the control voltage is appliedto the variable capacitance element and when a measurement of thecharacteristic of the circuit is acquired, the wait time being set to aplurality of values in accordance with the control voltage.
 12. Thecontrol method according to claim 11, wherein the wait time is changeddepending on a displacement of the control voltage.
 13. The controlmethod according to claim 11, wherein the wait time is changed dependingon the direction in which the control voltage is displaced.
 14. Thecontrol method according to claim 11, wherein the wait time is changeddepending on a displacement of the control voltage and the direction inwhich the control voltage is displaced.
 15. The control method accordingto claim 12, wherein the wait time is made proportional to thedisplacement of the control voltage.
 16. The control method according toclaim 13, wherein the wait time when the control voltage is deceased isset to be shorter than that when the control voltage is increased. 17.The control method according to claim 11, wherein the control voltageoutput section is connected to the variable capacitance element throughan AC-signal-blocking impedance element.
 18. The control methodaccording to claim 11, wherein the characteristic of the circuit is aresonant frequency or an impedance of the circuit.
 19. A control programthat controls a capacitance of a variable capacitance element, thecontrol program having steps to be executable by a computer, thecomputer including a storage section that stores a program and aprocessing section that expands and executes the stored program, thecontrol program comprising the steps of: setting a capacitance of avariable capacitance element by applying a control voltage with acontrol voltage output section that outputs a variable DC voltage; andacquiring a characteristic of a circuit containing the variablecapacitance element with a detection section that detects thecharacteristic of the circuit containing the variable capacitanceelement, wherein the detection section having a wait time between whenthe control voltage is applied to the variable capacitance element andwhen a measurement of the characteristic of the circuit is acquired, thewait time being set to a plurality of values in accordance with thecontrol voltage.
 20. The control program according to claim 19, whereinthe wait time is changed depending on a displacement of the controlvoltage.
 21. The control program according to claim 19, wherein the waittime is changed depending on the direction in which the control voltageis displaced.
 22. The control program according to claim 19, wherein thewait time is changed depending on a displacement of the control voltageand the direction in which the control voltage is displaced.
 23. Thecontrol program according to claim 20, wherein the wait time is madeproportional to the displacement of the control voltage.
 24. The controlprogram according to claim 21, wherein the wait time when the controlvoltage is deceased is set to be shorter than that when the controlvoltage is increased.
 25. The control program according to claim 19,wherein the control voltage output section is connected to the variablecapacitance element through an AC-signal-blocking impedance element. 26.The control program according to claim 19, wherein the characteristic ofthe circuit is a resonant frequency or an impedance of the circuit. 27.A semiconductor element comprising a storage section that stores thecontrol program according to claim
 19. 28. The semiconductor elementaccording to claim 27, further comprising a processing section thatloads and executes the control program stored in the storage section.